You have exhausted/emptied the fluid in the reservoir.

Unknowingly to you , the res has TWO containers with a wall between the two..one is for the front brakes and one for the rear..you have to fill the res all the way up for the fluid to go over the seperateing wall into the other container/cavity.

You do not see this other cavity from the outside.

This design is so that you have two brake systems and always have some brake when one side springs a leak..........dual circuit braking..as old as the hills

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A Dalton